Requirement for self-recognition by helper T cells has been analyzed in the T-cell dependent antibody response to TNP-KLH. Separation of T cells, B cells, and adherent accessory cells into three functionally distinct subpopulations has allowed helper T cell recognition of MHC determinants expressed on macrophages to be independently assessed from T cell recognition of B cell expressed MHC determinants. Helper T cells were obtained from parent yields F1 and F1 yields parent radiation bone marrow chimeras to avoid complicating allogeneic effects. The results of such experiments demonstrated: 1) Helper T cell recognition of MHC determinants on macrophages, not B cells; 2) the self-specificity of helper T cells is determined by the MHC environment in which T cells mature, and not by their genotype.